FirstLight IV: Diversity in sub-L$_*$ galaxies at cosmic dawn

Abstract

Using a large sample of sub-L * galaxies, with similar UV magnitudes, M UV −19 at 𝑧 6, extracted from the FirstLight simulations, we show the diversity of galaxies at the end of the reionization epoch. We find a factor ∼40 variation in the specific star-formation rate (sSFR). This drives a ∼1 dex range in equivalent width of the [OIII]𝜆5007 line. Variations in nebular metallicity and ionization parameter within HII regions lead to a scatter in the equivalent widths and [OIII]/H𝛼 line ratio at a fixed sSFR. [OIII]-bright emitters have higher ionization parameters and/or higher metallicities than H𝛼-bright galaxies. According to the surface brightness maps in both [OIII] and H𝛼, [OIII]-bright emitters are more compact than H𝛼-bright galaxies. H𝛼 luminosity is higher than [OIII] if star formation is distributed over extended regions. OIII dominates if it is concentrated in compact clumps. In both cases, the H𝛼-emitting gas is significantly more extended than [OIII].